The Environmental Protection Agency’s little-known Office of Homeland Security has illegally blocked investigations by an independent watchdog for years, an inspector with the agency’s Office of Inspector General will tell Congress Wednesday.

Patrick Sullivan, the assistant EPA inspector general for investigations, is expected to testify Wednesday before a House oversight committee about the activities of the Office of Homeland Security, a unit run by President Obama’s political staff.

The office of about 10 employees is overseen by EPA Administrator Gina McCarthy’s office, and the inspector general’s office is accusing it of operating illegally as a “rogue law enforcement agency” that has impeded independent investigations into employee misconduct, computer security and external threats, including compelling employees involved in cases to sign non-disclosure agreements.

“Under the heavy cloak of ‘national security,’ the Office of Homeland Security has repeatedly rebuffed and refused to cooperate with the OIG’s ongoing requests for information or cooperation,” Sullivan wrote in prepared testimony. “This block unquestionably has hamstrung the Office of Inspector General’s ability to carry out its statutory mandate to investigate wrongdoing of EPA employees.”

EPA Deputy Administrator Bob Perciasepe was expected to tell Congress that the agency’s employees work cooperatively with the inspector general and support its mission, according to his prepared testimony.

Perciasepe assured Congress in his prepared testimony that the EPA remains committed to ensuring that the inspector general’s office successfully roots out waste, fraud and abuse across the agency.

The inspector general, Arthur A. Elkins, Jr., was appointed to lead the office by Obama in 2010. However, it’s an independent office within the agency expected to be outside of political influence.

“It’s disturbing that even investigations by this administration’s own nonpartisan watchdogs are being blocked by political appointees,” said House Oversight and Government Reform Committee Chairman Darrell Issa, R-Calif.

EPA’s Office of Homeland Security was set up in 2003 by an administrative order, and has no statutory authority to conduct investigations or enforce the law, according to Sullivan’s testimony. Sullivan’s opinion was backed up by a staffer in the Office of Compliance and Enforcement Assurance, but the agency has not issued a legal opinion on the office’s role. Since July 2012, in an agreement with the FBI, it has been the primary contact on all investigations with a connection to national security.